Photolithography is a lithographic reproduction method in which patterns are applied on materials by means of exposure to radiation. Photolithography is typically used in printing technology and in semiconductor technology.
In semiconductor technology, photolithography is used to transfer structure information from a mask into a photoresist on a semiconductor substrate (for example a silicon wafer). After development of the latent image, the structure information can be transferred, for example by means of etching, into a layer of the semiconductor substrate that is arranged beneath the photoresist. The photoresist is then removed.
The repetition of this process sequence with different successive layers and precise alignment of the individual patterns with respect to one another is a technique used in the production of integrated circuits, so-called microchips.
In optical lithography, the image of a reticle (mask) is formed on a wafer using visible light by means of the interference of diffraction orders that are generated by the reticle pattern and are imaged by means of a lens. In accordance with the prior art, unpolarized light is generally used for photolithographic applications. The resolution capability of an optical imaging system is described by the so-called NA value (numerical aperture), the NA value of known imaging lenses typically lying in the range of between 0.8 and 0.85. In order to achieve better resolutions, higher NA values are sought, which leads to a higher angular range of electromagnetic radiation that is incident on the wafer.
For small structure dimensions of objects on a reticle which are intended to be imaged on a semiconductor wafer, this results in high angles of the diffraction orders which have to be imaged by a lens of the lithography system. These high angles have the effect that different components of electromagnetic radiation radiated in with different polarization directions (for example referred to as TE-polarized component, “transverse electric”, and as TM-polarized component, “transverse magnetic”) form a different contrast on the plane of the wafer. TM-polarized light forms a poor contrast at high angles. In order to avoid TM polarization, it has been proposed that only TE-polarized light could be used for exposure.
However, for small structure dimensions, that is to say for small pitches to be imaged, the reticle itself functions as a polarizer that preferably transmits TM-polarized light without transmission losses, whereas high transmission losses occur in the case of TE-polarized light. As a result, the intensity of the TE-polarized light is reduced, which light generates better images on the wafer than the TM-polarized light that is transmitted essentially in an unattenuated manner.
In known lithography arrangements, particularly for small structure dimensions, these effects lead to quality problems in the imaging of structures from a reticle onto a wafer and may adversely influence the functionality of integrated circuits on the wafer.
DE 101 24 566 A1 describes an optical imaging system comprising polarization means, which has a polarization rotating element for rotating radial polarization into tangential polarization, the polarization rotating element being arranged at a predeterminable location in the region from the imaging-functional optical components following the radial polarization generating means.
U.S. Pat. No. 6,310,679 B1 describes a projection exposure arrangement, in which a device for rotating the polarization direction is arranged between the mask and the substrate, the polarization being rotated with the objective of achieving an increase in the depth of focus.
US 2004/0119954 A1 discloses an immersion exposure arrangement, in which a recurring pattern formed on a mask is imaged onto an object by means of an optical exposure system in such a way that the light used for imaging is exclusively s-polarized in a predetermined range of the angle of incidence.